FrameCollection: fix array_filter mutating the array to be assoc, close

filp#550

src/Whoops/Exception/FrameCollection.php

@@ -42,7 +42,7 @@ public function __construct(array $frames)
      */
     public function filter($callable)
     {
-        $this->frames = array_filter($this->frames, $callable);
+        $this->frames = array_values(array_filter($this->frames, $callable));
         return $this;
     }